Iodine-catalyzed regioselective thiolation of imidazo[1,2-a]pyridines using sulfonyl hydrazides as a thiol surrogate

Article abstract of DOI:10.1039/c5ob00033e

Iodine-catalyzed regioselective sulfenylation of imidazo[1,2-a]pyridines via C(sp²)-H bond functionalization has been achieved using sulfonyl hydrazides as a thiol surrogate. A library of 3-sulfanylimidazopyridines with broad functionalities was synthesized under metal and oxidant-free practical reaction conditions. This methodology is also applicable for the regioselective sulfenylation of imidazo[2,1-b]thiazole and benzo[d]imidazo[2,1-b]thiazole. This journal is

Full text of DOI:10.1039/c5ob00033e

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Iodine-catalyzed regioselective thiolation of imidazo[1,2-a]pyridines
using sulfonyl hydrazides as a thiol surrogate
Avik Kumar Bagdi,a Shubhanjan Mitra,a Monoranjan Ghosh,a and Alakananda Hajra*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Iodine-catalyzed regioselective sulfenylation of imidazo[1,2-a]pyridines via C(sp2)-H bond
functionalization has been achieved using sulfonyl hydrazides as a thiol surrogate. A library of 3-
sulfanylimidazopyridines with broad functionalities was synthesized under metal and oxidant-free
practical reaction conditions. This methodology is also applicable for the regioselective sulfenylation of
10 imidazo[2,1-b]thiazole and benzo[d]imidazo[2,1-b]thiazole.
value against variety of diseases.9 Imidazo[1,2-a]pyridines
bearing a thioether side chain at 3 position are highly active
against human cytomegalovirus and varicella-zoster virus.10
50 However, only few methodologies have been developed for the
sulfenylation of imidazo[1,2-a]pyridine moieties.11 Thiols and
disulfides have been used as sulphenylating agents in all these
cases. The uses of these reagents have some practical limitations
such as thiols are foul-smelling, volatile, toxic and disulfides are
55 expensive and moisture sensitive. In addition, these
methodologies suffer from some drawbacks like use of transition
metal catalyst, stoichiometric amount of oxidants, bases, harsh
reaction conditions, and limited applicability. Employment of
transition metal catalysts and oxidants generate hazardous wastes
60 which are not desirable from the aspect of green environment.
Thus, it would be highly desirable to develop an alternate way for
the sulfenylation of imidazo[1,2-a]pyridines under transition
metal and oxidant-free conditions. In search for an alternate
sulfenylating agent with desired synthetic features, we have
65 turned our attention to sulfonyl hydrazides. Ready accessibility
and practical usability of sulfonyl hydrazides prompted us to
investigate the employment of these reagents as thiol surrogate
for the thiolation of imidazopyridines (Scheme 1).
1. Introduction
Development of new methodologies for the C-S bond formation
has gained much attention in organic synthesis due to the
occurrences of C-S bond in biologically and pharmaceutically
15 active compounds.1 The organosulfur compounds are also
important for the synthesis of functional materials.2 Among the
organosulfur compounds, aryl and heteroaryl sulfides and their
derivatives have wide applications in organic synthesis,
pharmaceutical industry as well as in material science.3 Direct
20 formation of C-S bond via C-H functionalization of heterocycles
have received considerable attention in recent years, as a
potentially more efficient and complementary process to the
conventional cross-coupling methodology.4 Thiols, disulfides, S-
arylthiophthalimides, sulfonyl chlorides, sodium sulfinates etc
25 have been employed as the sulfenylating agents. Very recently,
sulfonyl hydrazide has emerged as
a new and efficient
sulfenylating agent through the cleavage of sulfur-oxygen and
sulfur-nitrogen bonds.5 These are stable solids, odorless, easy to
handle and readily accessible compounds. Tian and coworkers
30 first reported the thiolation of indoles employing sulfonyl
hydrazides as thiol surrogate.5a Subsequently, Singh et al also
reported the synthesis of vinyl sulfides and diaryl thioethers using
sulfonyl hydrazides under microwave irradiation.5c,d So far, the
employments of sulfonyl hydrazides as sulfenylating agent are
35 limited.
Imidazopyridines are one of the nitrogen containing important
fused heterocycles which are found in various marketed drugs
and compounds with biological impact.6 These are also very
useful in material science.7 So, continuous efforts have been paid
40 on the syntheses and functionalizations of the imidazopyridine
derivatives.8 The pharmaceutical property of imidazo[1,2-
a]pyridine is critically dependent on the nature of the substituents
at 3 positions. Incorporation of sulfenyl groups in heterocycle
could impart marked biological properties to derivatives like 3-
70
Scheme 1 Thiolation of imidazo[1,2-a]pyridines by different
sulphenylating reagents
45 sulfenyl
indoles,
3-sulfenyl
pyrroles,
and
3-
Due to immense importance of imidazopyridines we are also
sulfenylimidazopyridines which are of considerable therapeutic
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Table 1: Optimization of the reaction conditionsa
interested about the chemistry of these derivatives. Recently, we
have reported various strategies for the synthesis of
functionalized imidazopyridine derivatives employing easily
accessible chemicals.12 Based on our experiences on imidazo[1,2-
a]pyridines and inspired by Tian’s pioneering work, we
envisaged that imidazo[1,2-a]pyiridines could be sulfenylated at
3-position under the practical reaction conditions. Herein, we
report a new iodine-catalyzed convenient and regioselective
protocol for the sulfenylation of imidazo[1,2-a]pyridines
5
Entry Catalyst [mol%]
Solvent
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
DCE
Toluene
Dioxane
THF
DMSO
DMF
MeCN
EtOAc
-
Ethanold
Ethanol
Ethanol
Temperature Yield [%]b
1
2
I2 (10)
I2 (10)
I2 (10)
I2 (15)
I2 (20)
I2 (25)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
TBAI (20)
NIS (20)
60 ºC
70 ºC
refluxc
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
70 ºC
44
64
66
75
82
83
66
42
38
48
47
33
36
24
68
87
-
3
4
5
6
7
8
9
10 employing sulfonyl hydrazides as thiol surrogate in ethanol at 70
ºC (Scheme 2).
R1
R2
R1
O
S
Cat. I2
N
N
N
R2
NHNH2
+
N
10
11
12
13
14
15
16
17
18
Ethanol
O
S
Ar
Ar
1
2
3
No Oxidant
No Base
Sulfenation through cleavage of S-O and S-N bonds
Scheme 2 Regioselective thiolation of imidazo[1,2-a]pyridines
44
15 by sulfonyl hydrazides
a Carried out with 0.5 mmol of 1a and 0.6 mmol of 2a in the presence of
45 catalyst in solvent (0.5 mL) at 70 ºC for 10 h. b Isolated yields. c Reaction
proceeded under refluxing condition. d Only few drops of ethanol (0.1
mL) was used.
2. Results and discussion
The reaction between 2-phenylimidazo[1,2-a]pyridine and p-
20 toluenesulfonyl hydrazide was taken as the model reaction to
optimize the reaction conditions (Table 1). Initially the reaction
was carried out using 10 mol% I2 as catalyst at 60 ºC.
Gratifyingly the desired product was obtained with 44% yield
after 10 h (Table 1, entry 1). Inspired by this result the reaction
25 temperature was further increased and higher yield was obtained
at 70 ºC (Table 1, entries 2-3). After that, catalyst loading was
screened to improve the yield. On increasing the catalyst loading
to 15, 20, 25 mol%; the desired product obtained with 75, 87,
88% (Table 1, entries 4-6). Thus the catalyst loading was
30 optimized at 20 mol%. Then reaction was performed in different
common solvents like MeOH, DMSO, DMF, MeCN etc (Table 1,
entries 7-15). However no improvement in yields was obtained
compared to the EtOH. The reaction proceeded well under
solvent-free conditions to afford the product with 68%. For
35 further improvement of the yield, the reaction was carried out
using minimum amount of ethanol and in that case the desired
product was obtained in 87% yield (Table 1, entry 16). Other
iodide sources like TBAI, NIS were not effective like I2 (Table 1,
entries 17-18). Thus the optimized yield (87%) was obtained in
40 presence of 20 mol% I2 in ethanol (only few drops) at 70 ºC
(Table 1, entry 16).
With the optimized reaction conditions in hand, substrate
scopes of this protocol were investigated and the results are
50 represented in table 2. At first various imidazo[1,2-a]pyridines
were introduced with p-toluenesulfonyl hydrazide under
optimized reaction conditions. Imidazo[1,2-a]pyridines bearing
various substituents like -Me, -Cl at different positions on the
pyridine ring efficiently reacted with p-toluenesulfonyl hydrazide
55 to afford the desired products with high to excellent yields (3aa,
3ba, 3ca and 3da). The imidazo[1,2-a]pyridines with electron-
donating groups (-Me, -OMe) on the phenyl ring were well
tolerated (3ea and 3fa). The imidazo[1,2-a]pyridines bearing 2’-
hydroxy phenyl substituent produced the desired product with
60 high yields (3ga). Halogens (-Cl, -F) containing imidazopyridines
afforded the corresponding products with high yields (3ha and
3ia). Strong electron-withdrawing groups like -NO2, -CF3, -CN in
the phenyl ring also successfully gave the desired products
without any difficulties (3ja, 3ka and 3la). Imidazo[1,2-
65 a]pyridines with heteroaryl substituent also well tolerated under
the optimized reaction conditions (3ma). Based on the
optimization study, the scope of the sulfonyl hydrazides was also
studied. The nature of the sulfonyl hydrazides did not have a
great effect on the results. When benzenesulfonyl hydrazide was
70 used as reagent, the corresponding product 3bb was formed in
84% yield. Sulfonyl hydrazides with different substituents (-
OMe, -Cl) on the phenyl ring afforded almost comparable yields
(3bc and 3ad). It is noteworthy that the aliphatic sulfonyl
hydrazides also gave the corresponding products with good yields
75 (3be, 3ce and 3bf).
2
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Table 2 Substrates scope for the reaction of imidazopyridines 1 with
sulfonyl hydrazides 2a
were obtained with excellent yields (5aa and 7aa) (Scheme 4). It
is also notable that only single regioisomer was obtained in case
20 of imidazo[2,1-b]thiazole (4a) having different nucleophilic sites.
Scheme 4 Regioselective thiolation of imidazoheterocycles
The yield of the reaction (3aa, 85%) did not decrease
significantly in presence of the radical scavenger TEMPO (3
25 equiv.) which indicates that the reaction did not take place
through a radical pathway. Based on this experiment and the
literature reports5a a plausible mechanism for the sulfenylation of
imidazopyridines has been represented in Scheme 5. Initially,
sulfenyl iodide (I) is formed by the reaction between sulfonyl
30 hydrazide and I2 along with the elimination of HOI, H2O, and N2.
The imidazolenium intermediate II is formed by the subsequent
electrophilic attack by the sulfur electrophile (I) at the 3-position
of the imidazopyridine moieties. Finally, the intermediate II
afforded the desired product by the elimination of HI. This HI
35 reacts with HOI to regenerate the catalyst I2.
5
a Reaction conditions: 0.5 mmol of 1 and 0.6 mmol of 2 in the presence of
I2 (20 mol%) in EtOH (0.1 mL) at 70 ºC for 10 h, Isolated yields.
The gram-scale reaction was also performed in the usual
laboratory setup by taking the reaction of 2-phenylimidazo[1,2-
a]pyridine with p-toluenesulfonyl hydrazide (Scheme 3). This
10 experiment clearly demonstrated the practical applicability of this
new protocol for the sulfenylation of imidazopyridines.
Scheme 5 A plausible reaction mechanism
Scheme 3 Synthetic application: Gram-scale reaction
3. Conclusions
In conclusion, we have developed a new strategy for the
40 regioselective sulfenylation of imidazo[1,2-a]pyridines through
I2-catalyzed direct functionalization of sp2 C-H bond. Use of
sulfonyl hydrazides as thiol surrogate has been demonstrated in
Next we turned our focus to apply this protocol for other
15 imidazoheterocycles.
So,
imidazo[2,1-b]thiazole
and
benzo[d]imidazo[2,1-b]thiazole were tested under the optimized
reaction conditions. To our delight the corresponding products
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this reaction which occurs through the cleavage of sulfur-nitrogen
76.28; H, 5.39; N, 8.39%.
and sulfur-oxygen bonds. This methodology is applicable to both 60 7-Methyl-2-phenyl-3-(p-tolylthio)H-imidazo[1,2-a]pyridine
1
alkyl and aryl sulfonyl hydrazides, and various imidazopyridines
with broad functionalities. Operational simplicity, transition
metal and oxidant-free, and environmentally benign reaction
conditions, scalable are the attractive features of the present
(3ca): Isolated yield: 86%; IR: 3045, 2923, 1645, 1485 cm-1; H
NMR (400 MHz, CDCl3): δ 8.19-8.17 (m, 2H), 8.12 (d, J = 6.8
Hz, 1H), 7.48 (s, 1H), 7.44-7.40 (m, 2H), 7.37-7.33 (m, 1H),7.00
(d, J = 8.0 Hz, 2H), 6.90-6.88 (m, 2H), 6.68-6.66 (m, 1H), 2.41
5
protocol. This methodology is also applicable to imidazo[2,1- 65 (s, 3H), 2.24 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 151.1,
b]thiazole and benzo[d]imidazo[2,1-b]thiazole moieties. We
147.5, 137.9, 136.0, 133.6, 131.9, 130.2, 128.5, 128.4, 128.4,
125.8, 123.7, 116.2, 115.7, 106.1, 21.5, 20.9; Anal. Calcd for
C21H18N2S: C, 76.33; H, 5.49; N, 8.48%. Found: C, 76.21; H,
5.38; N, 8.34%.
believe that our protocol is a versatile and practical alternative to
10 the existing synthetic methodologies for the sulfenylation of
imidazoheterocycles.
70 6-Chloro-2-phenyl-3-(p-tolylthio)H-imidazo[1,2-a]pyridine
(3da): Isolated yield: 76%; IR: 3045, 2921, 1595, 1488, 1080 cm-
1; H NMR (400 MHz, CDCl3): δ 8.32-8.31 (m, 1H), 8.21-8.18
(m, 2H), 7.66-7.63 (m, 1H), 7.45-7.35 (m, 3H), 7.28-7.25 (m,
Experimental
General Methods and Materials
15 1H NMR spectra were determined on a Bruker 400 (400 MHz)
1
spectrometer as solutions in CDCl3. Chemical shifts are
1H), 7.03 (d, J = 8.0 Hz, 2H), 6.92-6.90 (m, 2H), 2.26 (s, 3H);
expressed in parts per million (δ) and are referenced to 75 13C NMR (100 MHz, CDCl3): δ 152.0, 145.4, 136.5, 133.1,
tetramethylsilane (TMS) as internal standard and the signals were
reported as s (singlet), d (doublet), t (triplet), m (multiplet) and
20 coupling constants J were given in Hz. 13C NMR spectra were
recorded at 100 MHz in CDCl3 solution. All reactions were
131.0, 130.4, 128.9, 128.6, 128.4, 128.1, 126.0, 122.6, 121.6,
118.1, 107.9, 21.0; Anal. Calcd for C20H15ClN2S: C, 68.46; H,
4.31; N, 7.98%. Found: C, 68.32; H, 4.20; N, 7.81%.
2-p-tolyl-3-(p-tolylthio)H-imidazo[1,2-a]pyridine
(3ea):
carried out in open air conditions. TLC was done on silica gel 80 Isolated yield: 86%; IR: 3045, 2920, 1635, 1480 cm-1; H NMR
1
coated glass slide (Merck, Silica gel G for TLC). Silica gel (60-
120 mesh, SRL, India) was used for column chromatography.
25 Petroleum ether refers to the fraction boiling in the range of 60-
(400 MHz, CDCl3): δ 8.24 (d, J = 6.8 Hz, 1H), 8.11 (d, J = 8.4
Hz, 2H), 7.70 (d, J = 8.8 Hz, 1H), 7.29-7.22 (m, 3H), 6.99 (d, J =
8.0 Hz, 2H), 6.89 (d, J = 8.4 Hz, 2H), 6.80 (t, J = 6.8 Hz, 1H),
2.36 (s, 3H), 2.23 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 151.2,
o
80 C unless otherwise mentioned. All solvents were dried and
distilled before use. Solvents, reagents and chemicals were 85 147.0, 138.5, 136.0, 131.6, 130.6, 130.2, 129.2, 128.3, 126.5,
purchased from Aldrich. All the imidazopyridines were
synthesized by following reported methodology.12d,f
125.9, 124.5, 117.5, 112.9, 106.6, 21.4, 20.91; Anal. Calcd for
C21H18N2S: C, 76.33; H, 5.49; N, 8.48%. Found: C, 76.30; H,
5.33; N, 8.40%.
30
Typical procedure for the synthesis of 2-phenyl-3-p-
tolylsulfanyl-imidazo[1,2-a]pyridine (3aa)11d
2-(4-Methoxyphenyl)-3-(p-tolylthio)H-imidazo[1,2-a]pyridine
:
90 (3fa): Isolated yield: 79%; IR: 3035, 2929, 2351, 1612, 1477,
1
To a solution of 2-phenyl-imidazo[1,2-a]pyridine 1a (97 mg, 0.5
mmol) in ethanol (0.1 mL), p-toluene sulfonylhydrazide 2a (110
1026 cm-1; H NMR (400 MHz, CDCl3): δ 8.28-8.26 (m, 1H),
8.17-8.15 (m, 2H), 7.74 (d, J = 8.8 Hz, 1H), 7.35-7.31 (m, 1H),
7.02 (d, J = 8.4 Hz, 2H), 6.96 (d, J = 8.8 Hz, 2H), 6.92-6.85 (m,
3H), 1H), 3.83 (s, 3H), 2.25 (s, 3H); 13C NMR (100 MHz,
35 mg, 0.6 mmol) and I2 (20 mol-%) was added. The mixture was
o
stirred at 70 C for 10 h (TLC). After completion, the reaction
mixture was cooled to room temperature and ethanol was 95 CDCl3): δ 160.2, 150.8, 146.8, 136.2, 131.5, 130.3, 129.8, 126.9,
evaporated. Subsequently, the mixture was extracted with
dichloromethane (5 mL) followed by washing with brine (3 mL).
40 The combined organic phases were dried over anhydrous
Na2SO4. The crude product was concentrated in vacuo and was
125.9, 125.7, 124.6, 117.3, 114.0, 113.2, 106.2, 55.4, 21.0; Anal.
Calcd for C21H18N2OS: C, 72.80; H, 5.24; N, 8.09%. Found: C,
72.71; H, 5.11; N, 8.01%.
2-(3-(p-tolylthio)H-imidazo[1,2-a]pyridin-2-yl)phenol (3ga):
purified by column chromatography on silica gel using petroleum 100 Isolated yield: 80%; IR: 3587, 3024, 2926, 1620, 1485 cm-1; H
1
ether/ethylacetate (15:1 to 10:1) as eluent. Isolated yield: 87%;
IR: 3046, 2927, 1638, 1482 cm-1; 1H NMR (400 MHz, CDCl3): δ
45 8.18-8.17 (m, 1H), 8.13-8.11(m, 2H), 7.65-7.62 (m, 1H), 7.36-
7.32 (m, 2H), 7.30-7.20 (m, 2H), 6.92 (d, J = 8.0 Hz, 2H), 6.83-
NMR (400 MHz, CDCl3): δ 13.13 (brs, 1H), 8.63 (d, J = 8.0 Hz,
1H), 8.33 (d, J = 7.2 Hz, 1H), 7.62 (d, J = 9.2 Hz, 1H), 7.33 (t, J
= 8.0 Hz, 1H), 7.25 (t, J = 8.0 Hz, 1H), 7.05 (d, J = 8.4 Hz, 1H),
6.99 (d, J = 8.0 Hz, 2H), 6.93 (d, J = 8.4 Hz, 2H), 6.90-6.83 (m,
6.80 (m, 2H), 6.77-6.74 (m, 1H), 2.16 (s, 3H); 13C NMR (100 105 2H), 2.22 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 158.4, 149.2,
MHz, CDCl3): δ 151.2, 147.1, 136.1, 133.4, 131.5, 130.3, 128.6,
128.5, 126.7, 125.9, 124.6, 117.7, 113.1, 106.9, 20.9.
144.5, 136.4, 130.7, 130.6, 130.3, 127.8, 127.4, 126.2, 124.2,
118.9, 117.7, 116.6, 116.2, 113.7, 106.1, 20.9; Anal. Calcd for
C20H16N2OS: C, 72.26; H, 4.85; N, 8.43%. Found: C, 72.11; H,
4.73; N, 8.32%.
50 8-Methyl-2-phenyl-3-(p-tolylthio)H-imidazo[1,2-a]pyridine
1
(3ba): Isolated yield: 88%; IR: 3042, 2920, 1648, 1479 cm-1; H
NMR (400 MHz, CDCl3): δ 8.26 (d, J = 7.6 Hz, 2H), 8.17 (d, J = 110 2-(4-Chlorophenyl)-7-methyl-3-(p-tolylthio)H-imidazo[1,2-
6.8 Hz, 1H), 7.49-7.46 (m, 2H), 7.42-7.38 (m, 1H), 7.12 (d, J =
6.8 Hz, 1H), 7.04 (d, J = 8.0 Hz, 2H), 6.94 (d, J = 8.4 Hz, 2H),
55 6.77 (t, J = 6.8 Hz, 1H), 2.75 (s, 3H), 2.28 (s, 3H); 13C NMR (100
MHz, CDCl3): δ 150.8, 147.3, 135.9, 133.7, 131.8, 130.1, 128.5,
a]pyridine (3ha): Isolated yield: 89%; IR: 3048, 2930, 1598,
1
1472, 1083 cm-1; H NMR (400 MHz, CDCl3): δ 8.19-8.16 (m,
2H), 8.12 (d, J = 6.8 Hz, 1H), 7.39-7.36 (m, 2H), 7.08 (d, J = 6.8
Hz, 1H), 6.99 (d, J = 8.0 Hz, 2H), 6.86 (d, J = 8.4 Hz, 2H), 6.74
128.4, 127.6, 125.8, 125.4, 122.3, 113.0, 107.0, 20.9, 16.8; Anal. 115 (t, J = 6.8 Hz, 1H), 2.68 (s, 3H), 2.23 (s, 3H); 13C NMR (100
Calcd for C21H18N2S: C, 76.33; H, 5.49; N, 8.48%. Found: C,
MHz, CDCl3): δ 149.4, 147.3, 136.0, 134.2, 132.2, 131.4, 130.1,
4
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129.6, 128.5, 127.6, 125.7, 125.4, 122.2, 113.0, 107.2, 20.8, 16.7;
147.4, 135.5, 133.6, 129.3, 128.5, 128.4, 128.4, 127.6, 125.9,
Anal. Calcd for C21H17ClN2S: C, 69.12; H, 4.70; N, 7.68%. 60 125.5, 125.4, 122.2, 113.0, 106.4, 16.8.
Found: C, 69.02; H, 4.59; N, 7.52%.
3-(4-Methoxyphenylthio)-8-methyl-2-phenylH-imidazo[1,2-
2-(4-Fluorophenyl)-3-(p-tolylthio)H-imidazo[1,2-a]pyridine
(3ia): Isolated yield: 84%; IR: 3053, 2920, 2352, 1610, 1479 cm-
1; 1H NMR (400 MHz, CDCl3): δ 8.25-8.19 (m, 3H), 7.70 (d, J =
a]pyridine (3bc): Isolated yield: 81%; IR: 3041, 2928, 2343,
1620, 1469, 1020 cm-1; 1H NMR (400 MHz, CDCl3): δ 8.24–8.22
(m, 2H), 8.11 (d, J = 6.8 Hz, 1H), 7.43–7.39 (m, 2H), 7.35–7.33
5
8.8 Hz, 1H), 7.32-7.28 (m, 1H), 7.10 (t, J = 8.8 Hz, 2H), 7.00 (d, 65 (m, 1H), 7.01 (d, J = 6.8 Hz, 1H), 6.95–6.92 (m, 2H), 6.69–6.65
J = 8.0 Hz, 2H), 6.89-6.81 (m, 3H), 2.23 (s, 3H); 13C NMR (100
MHz, CDCl3): δ 163.1 (1JC-F = 247 Hz), 150.2, 146.9, 136.2,
(m, 3H), 3.61 (s, 3H), 2.66 (s, 3H); 13C NMR (100 MHz, CDCl3):
δ 158.4, 150.2, 146.9, 133.5, 128.5, 128.3, 128.3, 127.8, 127.4,
125.5, 125.4, 122.1, 115.0, 112.9, 108.0, 55.1, 16.7; Anal. Calcd
for C21H18N2OS: C, 72.80; H, 5.24; N, 8.09%. Found: C, 72.71;
70 H, 5.13; N, 7.98%.
10 131.3, 130.3, 130.2, 129.5, 126.8, 125.8, 124.5, 117.5, 115.4 (2JC-
= 22 Hz), 113.1, 106.7, 20.9; Anal. Calcd for C20H15FN2S: C,
F
71.83; H, 4.52; N, 8.38%. Found: C, 71.74; H, 4.43; N, 8.26%.
2-(3-Nitrophenyl)-3-(p-tolylthio)H-imidazo[1,2-a]pyridine
(3ja): Isolated yield: 78%; IR: 3083, 2929, 1620, 1519, 1342 cm-
15 1; 1H NMR (400 MHz, CDCl3): δ 9.15-9.14 (m, 1H), 8.59 (d, J =
8.0 Hz, 1H), 8.34 (d, J = 7.2 Hz, 1H), 8.21-8.18 (m, 1H), 7.74 (d,
3-(4-Chlorophenylthio)-2-p-tolylH-imidazo[1,2-a]pyridine
(3ad): Isolated yield: 86%; IR: 3046, 2928, 1580, 1482, 1083 cm-
1
1; H NMR (400 MHz, CDCl3): δ 8.22 (d, J = 6.8 Hz, 1H), 8.05
(d, J = 8.4 Hz, 2H), 7.75 (d, J = 9.2 Hz, 1H), 7.35–7.31 (m, 1H),
J = 9.2 Hz, 1H), 7.58 (t, J = 8.0 Hz, 1H), 7.40-7.36 (m, 1H), 7.03 75 7.24 (d, J = 8.0 Hz, 2H), 7.18–7.14 (m, 2H), 6.92–6.85 (m, 3H),
(d, J = 8.0 Hz, 2H), 6.94-6.90 (m ,3H), 2.25 (s, 3H); 13C NMR
(100 MHz, CDCl3): δ 148.6, 148.2, 147.1, 136.7, 135.3, 134.1,
20 130.7, 130.4, 129.4, 127.3, 126.3, 124.7, 123.3, 123.1, 117.9,
113.7, 108.6, 21.0; Anal. Calcd for C20H15N3O2S: C, 66.46; H,
4.18; N, 11.63%. Found: C, 66.31; H, 4.05; N, 11.50%.
2.38 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 151.7, 147.2, 138.8,
133.9, 132.1, 130.2, 129.6, 129.3, 128.3, 126.9, 126.9, 124.3,
117.7, 113.3, 105.5, 21.4; Anal. Calcd for C20H15ClN2S: C,
68.46; H, 4.31; N, 7.98%. Found: C, 68.33; H, 4.22; N, 7.84%.
80 3-(Methylthio)-2-phenylH-imidazo[1,2-a]pyridine
(3be):
1
3-(p-tolylthio)-2-(4-(trifluoromethyl)phenyl)H-imidazo[1,2-
Isolated yield: 63%; IR: 3057, 2918, 1608, 1471 cm-1; H NMR
(400 MHz, CDCl3): δ 8.41 (d, J = 8.8 Hz, 1H), 8.21–8.19 (m,
2H), 7.61 (d, J = 8.8 Hz, 1H), 7.43–7.39 (m, 2H), 7.34–7.31 (m,
1H), 7.25–7.21 (m, 1H), 6.89–6.86 (m, 1H), 2.19 (s, 3H); 13C
a]pyridine (3ka): Isolated yield: 75%; IR: 3040, 2927, 1588,
1
25 1475 cm-1; H NMR (400 MHz, CDCl3): δ 8.34 (d, J = 8.0 Hz,
2H), 8.14 (d, J = 6.8 Hz, 1H), 7.66 (d, J = 8.4 Hz, 2H), 7.47 (s,
1H), 7.00 (d, J = 8.0 Hz, 2H), 6.88 (d, J = 8.0 Hz, 2H), 6.69 (d, J 85 NMR (100 MHz, CDCl3): δ 148.7, 146.3, 133.7, 128.6, 128.5,
= 6.4 Hz, 1H), 2.41 (s, 3H), 2.24 (s, 3H); 13C NMR (100 MHz,
128.4, 128.4, 126.2, 124.4, 117.6, 113.0, 18.3.
CDCl3): δ 149.3, 147.5, 138.3, 137.2, 136.3, 131.3, 130.3, 130.1
7-Methyl-3-(methylthio)-2-phenylH-imidazo[1,2-a]pyridine
(3ce): Isolated yield: 62%; IR: 3051, 2915, 1612, 1454 cm-1; H
30 (2JC-F = 32 Hz), 128.5, 125.8, 125.3 (3JC-F = 4Hz), 124.3 (1JC-F
=
1
271 Hz), 123.8, 116.3, 116.1, 107.2, 21.4, 20.9; Anal. Calcd for
NMR (400 MHz, CDCl3): δ 8.27 (d, J = 7.2 Hz, 1H), 8.20–8.18
C21H15F3N2S: C, 65.61; H, 3.93; N, 7.29%. Found: C, 65.47; H, 90 (m, 2H), 7.39 (t, J = 7.6 Hz, 2H), 7.34 (s, 1H), 7.31–7.30 (m, 1H),
3.84; N, 7.16%.
6.70–6.67 (m, 1H), 2.36 (s, 3H), 2.16 (s, 3H); 13C NMR (100
MHz, CDCl3): δ 148.7, 146.8, 137.2, 134.0, 128.4, 128.3, 128.2,
123.5, 116.2, 115.4, 21.4, 18.4.
4-(3-(p-tolylthio)H-imidazo[1,2-a]pyridin-2-yl)benzonitrile
35 (3la): Isolated yield: 74%; IR: 3072, 2925, 2225, 1616, 1483,
1346 cm-1; 1H NMR (400 MHz, CDCl3): δ 8.37 (d, J = 8.4 Hz,
3-(Butylthio)-8-methyl-2-phenylH-imidazo[1,2-a]pyridine
2H), 8.28 (d, J = 6.8 Hz, 1H), 7.73-7.67 (m, 3H), 7.35 (t, J = 8.0 95 (3bf): Isolated yield: 70%; IR: 3045, 2912, 1640, 1462 cm-1; H
1
Hz, 1H), 7.02 (d, J = 8.0 Hz, 2H), 6.91-6.87 (m, 3H), 2.24 (s,
3H); 13C NMR (100 MHz, CDCl3): δ 148.5, 147.1, 137.9, 136.5,
40 132.1, 130.6, 130.4, 128.6, 127.2, 125.9, 124.6, 118.9, 117.8,
113.6, 111.7, 108.5, 20.9; Anal. Calcd for C21H15N3S: C, 73.87;
H, 4.43; N, 12.31%. Found: C, 73.79; H, 4.36; N, 12.24%.
NMR (400 MHz, CDCl3): δ 8.34 (d, J = 6.8 Hz, 1H), 8.28 (d, J =
7.2 Hz, 2H), 7.45 (t, J = 7.6 Hz, 2H), 7.35 (t, J = 7.6 Hz, 1H),
7.04 (d, J = 6.8 Hz, 1H), 6.79 (t, J = 6.8 Hz, 1H), 2.66 (s, 3H),
2.60 (t, J = 7.2 Hz, 2H), 1.41–1.35 (m, 2H), 1.33–1.24 (m, 2H),
100 0.73 (t, J = 7.2 Hz, 3H).; 13C NMR (100 MHz, CDCl3): δ 149.0,
146.5, 134.0, 128.5, 128.2, 128.0, 127.4, 124.8, 122.2, 112.7,
110.7, 35.4, 31.4, 21.7, 16.8, 13.5; Anal. Calcd for C18H20N2S: C,
72.93; H, 6.80; N, 9.45%. Found: C, 72.79; H, 6.74; N, 9.34%.
Typical procedure for the synthesis of 6-phenyl-5-(p-
7-Methyl-2-(thiophen-2-yl)-3-(p-tolylthio)H-imidazo[1,2-
a]pyridine (3ma): Isolated yield: 71%; IR: 3045, 2927, 1641,
1
45 1487 cm-1; H NMR (400 MHz, CDCl3): δ 8.21 (d, J = 8.8 Hz,
1H), 8.00-7.99 (m, 1H), 7.65 (d, J = 8.8 Hz, 1H), 7.34-7.32 (m,
1H), 7.27-7.23 (m, 1H), 7.08–7.06 (m, 1H), 6.98–6.92 (m, 4H), 105 tolylthio)imidazo[2,1-b]thiazole (5aa):
6.80–6.76 (m, 1H), 2.21 (s, 3H); 13C NMR (100 MHz, CDCl3): δ
146.9, 146.5, 136.3, 136.2, 130.9, 130.1, 127.7, 126.7, 126.5,
50 126.2, 124.3, 117.3, 113.0, 106.1, 20.9; Anal. Calcd for
C19H16N2S2: C, 67.82; H, 4.79; N, 8.33%. Found: C, 67.71; H,
4.68; N, 8.25%.
A mixture of 2-phenyl-imidazo[1,2-a]thiazole 4a (100 mg, 0.5
mmol) and p-toluene sulfonylhydrazide 2a (110 mg, 0.6 mmol)
was stirred in presence of I2 (20 mol%) in ethanol (0.1 mL) at 70
0C for 10h (TLC). After completion, the reaction mixture was
110 cooled to room temperature and extracted with dichloromethane
(5 mL) followed by washing with brine (3 mL) and dried over
Na2SO4. After evaporation of solvent the crude product was
purified by column chromatography on silica gel using petroleum
ether/ethylacetate as eluent. Isolated yield: 88%; IR: 3041, 2920,
8-Methyl-2-phenyl-3-(phenylthio)H-imidazo[1,2-a]pyridine
(3bb)12f: Isolated yield: 84%; IR: 3040, 2923, 1643, 1479 cm-1;
55 1H NMR (400 MHz, CDCl3): δ 8.20–8.18 (m, 2H), 8.07 (d, J =
6.8 Hz, 1H), 7.42–7.38 (m, 2H), 7.34–7.30 (m, 1H), 7.16–7.12
(m, 2H), 7.08–7.02 (m, 2H), 6.97–6.94 (m, 2H), 6.68 (t, J = 7.2 115 1620, 1472 cm-1; 1H NMR (400 MHz, CDCl3): δ 8.11 (d , J = 7.2
Hz, 1H), 2.68 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 151.0,
Hz, 2H), 7.40–7.28 (m, 4H), 7.02–6.95 (m, 4H), 6.75 (d , J = 4.8
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DOI: 10.1039/C5OB00033E
Hz, 1H), 2.24 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 152.2,
151.2, 136.2, 133.6, 132.1, 130.2, 128.4, 128.1, 127.5, 126.2,
118.0, 112.9, 108.4, 20.9; Anal. Calcd for C18H14N2S2: C, 67.05;
H, 4.38; N, 8.69%. Found: C, 66.92; H, 4.29; N, 8.57%.
Chem. Commun., 2014, 50, 13121; (f) G. Kumaraswamy and R.
Raju, Adv. Synth. Catal., 2014, 356, 2591.
65 6 C. Enguehard-Gueiffier and A. Gueiffier, Mini Rev. Med. Chem.,
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(a) A. J. Stasyuk, M. Banasiewicz, M. K. Cyrański and D. T. Gryko,
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G.-F. Shi and Y. Cheng, J. Org. Chem., 2011, 76, 7458.
5
Typical procedure for the synthesis of 2-Phenyl-3-p-
tolylsulfanyl-benzo[d]imidazo[2,1-b]thiazole (7aa):
70 8 (a) A. K. Bagdi, S. Santra, K. Monir and A. Hajra, Chem. Commun.,
2014, DOI: 10.1039/C4CC08495K; (b) J. Koubachi, S. El
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A mixture of 2-phenyl-benzo[d]imidazo[2,1-b]thiazole 6a (125
mg, 0.5 mmol) and p-toluene sulfonylhydrazide 2a (110 mg, 0.6
mmol) was stirred in presence of I2 (20 mol%) in ethanol (0.1
10 mL) at 70 0C for 10h (TLC). After completion, the reaction
mixture was cooled to room temperature and extracted with
dichloromethane (5 mL) followed by washing with brine (3 mL)
and dried over Na2SO4. After evaporation of solvent the crude
product was purified by column chromatography on silica gel
15 using petroleum ether/ethylacetate as eluent. Isolated yield: 83%;
IR: 3048, 2926, 1614, 1464 cm-1; 1H NMR (400 MHz, CDCl3): δ
8.33 (d , J = 8.4 Hz, 1H), 8.11–8.09 (m, 2H), 7.65–7.63 (m, 1H),
7.44–7.37 (m, 2H), 7.34–7.21 (m, 3H), 7.07–7.02 (m, 4H), 2.23
(s, 3H); 13C NMR (100 MHz, CDCl3): δ 153.4, 150.6, 136.1,
20 133.5, 133.3, 132.9, 130.4, 130.2, 128.4, 128.3, 127.8, 126.4,
125.8, 124.9, 124.0, 114.5, 110.6, 21.0; Anal. Calcd for
C22H16N2S2: C, 70.93; H, 4.33; N, 7.52%. Found: C, 70.81; H,
4.24; N, 7.43%.
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90
25 Acknowledgements
A.H. acknowledges the financial support from CSIR, New Delhi
(Grant No. 02(0168)/13/EMR-II). M. G. thanks UGC for his
fellowship.
95
Notes and references
30 a Department of Chemistry, Visva-Bharati, Santiniketan-731235, India
E-mail: alakananda.hajra@visva-bharati.ac.in
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†Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
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